The present invention relates to a lock stitch sewing machine equipped with a horizontally rotating shuttle. More particularly, the present invention relates to a device for temporarily holding the severed end of a bobbin thread after the bobbin thread is cut such that the bobbin thread is securely engaged and crossed with a needle thread.
Referring to FIG. 11, one conventional way of holding a severed end of a bobbin thread after cutting will be explained. Numeral 101 denotes a throat plate, 102 denotes a movable blade, 103 denotes a bobbin, 104 denotes a stationary blade, 105 denotes a plate spring, and 107 denotes a shuttle. Symbol N denotes a needle, I.sub.1 denotes a bobbin thread, I.sub.2 denotes a needle thread.
When the stitching is finished, the movable blade 102 moves rightward and draws the bobbin thread I.sub.1 from the bobbin 103 by a predetermined length so the bobbin thread I.sub.1 is clamped between the movable blade 102 and the plate spring 105. As the movable blade 102 moves further rightward, the bobbin thread I.sub.1 is cut by the movable blade 102 and the stationary blade 104.
Since the severed end of the bobbin thread I.sub.1 is held between the movable blade 102 and the plate spring 105 as shown in FIG. 11 and FIG. 12, when the next stitching starts, a loop formed around the needle N passes over the bobbin 103 and crosses with the bobbin thread I.sub.1. Thus, the stitching is conducted.
Referring to FIG. 12, since the bobbin thread I.sub.1 is held between the movable blade 102 and the plate spring 105, the spring pressure of the plate spring 105 must be adjusted to meet the thread diameter and the friction coefficient of the thread. Accordingly, such adjustment work lowers sewing productivity.
If the bobbin thread diameter is small and the plate spring is adjusted to a large diameter bobbin thread, the bobbin thread will easily slip off of the plate spring in direction of arrows A, B in FIG. 12. Conversely, if the bobbin thread is large and the plate spring is adjusted to a small diameter bobbin thread, the bobbin thread I.sub.1 will be tightly clamped by the plate spring and will not be separated, and the subsequent stitching will be incomplete.